The conventional fluorescent lamp, namely a low pressure mercury vapor fluorescent electric discharge lamp, offers many advantages as a light source including high efficiency and good light distribution. However, control of the fluorescent lamp presents certain problems. Since it is a gas discharge device, a high starting voltage is required to initiate ionization and current limiting must be provided to avoid damage or destruction after ionization has taken place. Generally, the lamps are provided with a pair of heated electrodes to promote rapid starting, a transformer to provide high starting voltage, and a ballast is provided for current limiting.
There are several applications of fluorescent lighting in which dimming control, i.e. the reduction of light intensity from its normal running value, would be desirable. One present need for dimming control for fluorescent lamps arises especially in connection with fluorescent lighting systems on buses. In particular, certain present day intra-city buses provide a high level of illumination in the passenger areas and, under certain ambient lighting conditions, the internal bus lighting produces reflections on the windshield which makes it difficult for the driver to see out. Fluorescent lighting systems for buses are energized from a DC voltage source through an electronic inverter which supplies a pulsating DC output at relatively high voltage. It is desired to provide a light dimming device which will permit the bus driver to reduce the light intensity from the fluorescent lamps as needed by means of a simple manual control. There have been several attempts in the prior art to provide dimming devices for fluorescent lamps.
In AC fluorescent lighting systems, lamp dimming devices have been proposed which control the current conduction interval, or angle, for each half cycle of the supply voltage. A dimming system of this type using thyristors or silicon controlled rectifiers is described in U.S. Pat. No. 3,863,102 to Herzog and in U.S. Pat. No. 3,875,458 to Kappenhagen. A dimming system in which the current is switched on and off plural times for each half cycle by a gate controlled rectifier circuit is described in U.S. Pat. No. 3,422,309 to Spira et al. Another form of dimming circuit for AC fluorescent lighting circuits uses a saturable reactor in series with the ballast to adjust the voltage across the lamp. This type of dimmer is described in U.S. Pat. No. 3,264,518 to Stauverman.
In the typical DC fluorescent lamp circuit, an inverter is connected between the DC voltage source and the lamp. Inverters may be adapted to a wide range of DC source voltage, for example 5 to 50 volts, for producing a pulsating DC output voltage of sufficient amplitude to ionize the gas in a selected tube length of the fluorescent lamp. The inverters are typically operated at a frequency in the range from 10 kHz to 50 kHz. Usually, a ballast in the form of a series capacitor is connected between the inverter and the fluorescent lamp to limit the current flow through the ionized column of gas after the lamp is started. A transistor inverter circuit for fluorescent lamps using two transistors in push/pull configuration is described in U.S. Pat. No. 2,964,676 to Davies et al. Another transistor inverter for fluorescent lamps using a single transistor is described in U.S. Pat. No. 3,247,422 to Schultz.
Prior art dimming devices for DC fluorescent lamp circuits have obtained dimming by reducing the voltage across the lamp. In U.S. Pat. No. 3,119,048 to Tsuchiya, the fluorescent lamp is supplied by an inverter and dimming is obtained by reducing inverter output voltage by means of a variable feedback resistor. In U.S. Pat. No. 3,611,024 to Nakatsu et al., dimming is provided by reducing the lamp voltage by a variable resistor in series with the lamp discharge circuit or alternatively by control of the pulse amplitude from the pulse generator.
A general object of this invention is to provide an improved dimming means for controlling the brightness of a fluorescent lamp in a DC lighting circuit.